Film active matrix devices can be used for many purposes. One of the more common applications is as a display device. In this application, the device comprises a plurality of display elements arranged in an array formed by electrodes supported on opposing sides of a thin layer or film of electro-optical material and associated with switching means operable to control the application of data signals to the display elements in response to switching signals applied to the electrodes. A display device of this kind is suitable for displaying alphanumeric and video information using for the electro-optical layer materials like liquid crystal and electrophoretic suspensions. See, for example, the article by Lechner et al, PIEEE, November, 1981, pgs. 1566-1579, the contents of which are hereby incorporated by reference.
In known examples of this kind of liquid crystal matrix display device, the display elements are arranged in a matrix of rows and columns which are defined by respective pixel electrodes on one side of the active layer and opposing portions of a common electrode on the opposite side of the active layer. Switching means in the form of a transistor, for example a thin film transistor (TFT), is located adjacent the pixel electrode of its respective display element with its drain electrode connected to the pixel electrode. The source electrodes of all transistors in the same column are connected to a respective one of a set of column conductors to which data signals are applied, and the gate electrodes of all transistors in the same row are connected to a respective one of a set of row conductors to which switching (gating) signals are applied to switch the transistors on. The device is driven by repetitively scanning the row conductors one at a time in sequential fashion so as to turn on all transistors in each row in turn, while applying data signals to the column conductors appropriately in synchronism for each row in turn so as to build up a display. When the transistors are on, the data signals are supplied to the associated driving electrodes thus charging up the display elements. Each display element (LCD) or pixel as it is commonly called can be thought of as electrically equivalent to a capacitor. When the transistors are turned off, upon termination of the switching voltage, the charge is stored in the display elements concerned until the next time they are addressed with a scanning signal, in the case of a video display with non-interlaced scanning, in the next field period.
Display devices of this type are generally well known. Such an active matrix addressed liquid crystal display device may typically consist of 200,000 or more display elements and be capable of displaying TV pictures. The resolution of the displayed image depends upon the number of pixels forming the image. The trend has been to increase the number of pixels to, for example, a 480.times.640 matrix (total of 307,200 pixels) to achieve the resolution of a normal TV receiver. For large area display devices, the transistors used to drive the pixels are usually thin-film transistors (TFTs) deposited on a transparent substrate (glass or quartz). With the increasingly larger display areas now being proposed comes a corresponding increase in the number of display elements, and hence switching means, required.
A major problem in making such high resolution display devices is the number of connections required between the addressing circuitry and the TFT drivers for the pixels. Suppose, for example, that the active matrix is a 3 cm.times.4 cm rectangle, typical for projection television (PTV) applications. A matrix employing 480 rows and 640 columns would thus require 1120 connections to the addressing circuitry. The current state of the art does not allow so many connection points to be provided around the periphery of a 3.times.4 cm.sup.2 rectangular matrix without increasing the substrate area required per matrix, and thus the cost, significantly.
Another major problem in making large area display devices of this kind is yield. When using, for example, TFTs deposited on a transparent substrate with their associated row and column conductors, just a few defective pixel TFTs or one conductor break will render the device unacceptable. Depending on the nature of the defect, even one defective TFT can lead to one complete row and one complete column of display elements being unusable. In an attempt to overcome this problem, various redundancy schemes have been proposed. Takeda et al, Japan Displays '86, pps. 204-207, provides one additional TFT per pixel, so a total of two TFTs per pixel, with the TFTs controlled or driven by adjacent scanning lines. No additional gate or source lines are necessary. In a variation, three TFTs per pixel is described, the third TFT in this case interconnecting vertically adjacent pixels.
Takahashi et al, SID 87 Digest, pps. 79-81, focuses on line defects and proposes duplicate data input routes per line. In other words, each pixel is connected to two row conductors by way of a separate TFT, with each conductor driven from opposite sides. Duplicate or redundant lines are also described in Yamano et al, IEEE-TCE, Feb. 1985, pps. 39-43, though it is not clear whether extra TFTs for each pixel are provided.
In the redundancy schemes proposed so far to correct for line defects, duplicate or redundant lines have been provided, in some cases requiring additional TFTs per pixel to couple the redundant lines to each pixel. Also, driving of the conductive lines with the same signals from opposite sides of the matrix has been proposed.
As mentioned earlier, another problem is the large number of connections required between the active LCD display and the addressing circuitry. This is a difficult requirement to satisfy especially for projection systems, where the active LCD display leaves, for cost reasons, only a small narrow border region to accommodate the large number of required connections. Malmberg et al, SID 86 Digest, pps. 281-284, proposes to integrate the scanner electronics on the display substrate using the same technology used in the manufacture of the pixel drivers for the LCD elements, and further proposes to reduce the number of connections using a commutator or switch configuration based on the same matrix configuration used in the active display to select individual pixels. In the Malmberg proposal, the row lines are divided up into 16 sections of 8 lines each (for a 192.times.128 matrix). Operation for use as a TV display is not described, and external ICs are required to provide the data and select signals as shown in FIG. 7 of this publication.
This prior art proposal fails to recognize that the OFF-state for all non-selected row lines can and should be defined every TV line time, thereby eliminating the possibility that the row capacitors of non-selected rows will gradually build up during the frame time sufficient charge to allow the inputted information, video for a TV display, to be displayed at more than one line at a time. Another disadvantage is that the proposed switch configuration for the commutator makes it impossible (for the columns) or more difficult (for the rows) to incorporate efficient redundancy schemes.